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Synthesis and magnetism of ε-Fe3N submicrorods for magnetic resonance imaging† Peng Zhang, Xiaobai Wang, Wei Wang, Xiang Lei and Hua Yang* Fe3N submicrorods with desirable magnetic properties were prepared from a designed metal–organic framework precursor via a modified ethanediamine nitridation route. Magnetic resonance imaging using

Received 26th September 2015, Accepted 10th November 2015 DOI: 10.1039/c5dt03762j www.rsc.org/dalton

the Fe3N submicrorods was investigated carefully, confirming the Fe3N submicrorod could be acceptable as a significant contrast agent for its outstanding concentration dependent signal and high r2 relaxivity. Besides, the excellent saturation magnetization and homogeneous dispersity also developed the Fe3N application range.

Introduction Magnetic iron-based materials have been paid much attention due to their wide applications in data recording, superconductivity, magnetic fluids, microwave absorption etc.1–4 Especially, in medical diagnosis, iron-based materials applied as contrast agents for magnetic resonance imaging (MRI) have captured much interest in the diagnosis and photo-thermal therapy of cancer and tumours due to their superior magnetism and thermal effects.5–7 Conventional contrast agents were based on ferrite and metal iron at the nanoscale level, leading to limitations in their physical application. For instance, ferrite only provides limited imaging definition due to its limited magnetic properties,5,8,9 while metal iron failed in maintaining chemical stability, although a better magnetism was observed. Hence, an ideal contrast agent still needed further investigation. Iron nitride was acceptable as a significant candidate for a contrast agent in magnetic resonance imaging, due to its comparable magnetism to metal iron and excellent chemical stability. However, iron nitrides still have some boundaries in their physical applications when utilized as a contrast agent for MRI. Some important factors are the challenges of a harsh synthesis and morphology control. The conventional gas–solid ammonia technique prefers to trigger an agglomerate due to a high calcination temperature. And the physical techniques are favorable for film materials involving IBED,10 pulsed laser deposition,11 dc magnetron sputtering12 etc., which are rather adverse for a contrast agent. Besides, a continuous ammonia flow, the most common nitrogen source, involves high toxicity

College of Chemistry, Jilin University, Changchun, 130012, China. E-mail: [email protected] † Electronic supplementary information (ESI) available. See DOI: 10.1039/ c5dt03762j

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and high costs. A series of work on organic nitrogen sources decreased the great challenges of Fe3N preparation,13–16 but the morphology control was seldom discussed. Hence, it is rational to continue developing a more facile route to decrease this synthesis challenge. Moreover, morphology control could also improve the application of iron nitride. Herein, we synthesized ε-Fe3N submicrorods with good dispersion in water. MRI properties were investigated carefully, confirming the ε-Fe3N submicrorods could be regarded as an effective contrast agent.

Experimental section Preparation of the MOF precursor Firstly, 0.01 mol of oxalic acid was dissolved into 25 mL of N,N-dimethylacetamide (DMAC) with vigorous stirring, followed by 0.002 mol of ferrous chloride tetrahydrate quickly added. Next, 25 mL of water was added dropwise into the solution above and stirred for 30 min. Then the yellow suspension was centrifuged and washed with water and ethanol several times. Then the precipitate was dried for 12 h under the protection of a nitrogen flow at 50 °C from water and oxygen to collect the MOF precursor. The preparation scheme is listed in Fig. S1.† Preparation of ε-Fe3N submicrorods 0.1 g of the as-synthesized MOF precursors was transferred into a porcelain boat. Then 10 mL ethanediamine was added into another porcelain boat, and was placed in the top 2–3 cm of the ferrous oxalate precursor. Next, the furnace was heated to 540 °C for 1 h at a rate of 20 °C min−1 under a continuous nitrogen flow. After natural cooling, the ε-Fe3N microrods could be collected without any further treatment. The preparation scheme is listed in Fig. S2.†

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Characterization The X-ray diffraction (XRD) patterns were collected on a Shimadzu diffractometer X-6000 with Cu-Kα radiation (λ = 1.5406 Å). The magnetic properties were measured on a Lake Shore vibrating sample magnetometer 7404 (VSM), where the sample was fixed in a Teflon capsule without any magnetic contribution. Scanning electron microscopy (SEM) images were obtained on a S-4800 (Hitachi) equipped with an EDX spectrometer. The samples were mounted on aluminum studs by using a silicon substrate and sputtercoated with gold before analysis. The MRI properties at room temperature were measured on a Huantong 1.5T MR scanner for small animal imaging system (Shanghai, China) with the following parameters: TR = 2500 ms, TE = 19.2 ms.

Results and discussion The Fe3N submicrorods were prepared from a metal–organic framework (MOF) precursor via a modified ethanediamine nitridation. In Fig. 1, the powder X-ray diffraction (XRD) patterns of the product can be well indexed to ε-Fe3N, indicating the crystal evolution from ferrous oxalate to ε-Fe3N during the nitridation process. The diffraction peaks at 2θ = 38.13°, 41.05°, 43.52°, 57.27°, 68.92° and 70.50° can be well indexed to the (110), (002), (111), (112), (300) and (113) crystal planes of ε-Fe3N, corresponding to JCPDS no. 73-2101.17 The previous study from our group exhibits that the ethanediamine plays the crucial role of the nitrogen source, evaporating to complete the nitridation during the heating process.18 Thus, ethanediamine can effectively decrease the synthesis challenge for iron nitride, replacing the ammonia gas to realize highquality nitridation, which is favorable for decreasing the synthesis limitation effectively. The rapid heating ramp leads to the crystallization of Fe3N due to a sufficient nitrogen

Fig. 1 The XRD patterns of the MOF precursors (A), Fe3N submicrorods (B) and the standard diffraction pattern (C).

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source being preserved in a nitrogen-rich carbon matrix. The XRD patterns of samples calcinated at different heating rates are shown in the ESI.† Scanning electron microscopy (SEM) images are employed to investigate the morphologies of the MOF precursor and ε-Fe3N. As shown in Fig. 2, the MOF precursor shows a regular morphology of quadrangular submicrorods with a length of 5 μm and a diameter of 500 nm. After the calcination, the organic iron precursor tranforms into ε-Fe3N. The as-synthesized Fe3N maintains a similar submicrorod morphology to that of the MOF precursor, although the surface becomes rougher. Further investigation shows that the ε-Fe3N submicrorods consist of tiny Fe3N nanoparticles as shown in the detailed magnification in the ESI.† The tiny Fe3N nanoparticles have an average diameter of 10 nm as shown in Fig. S4.† We believe that this particular morphology is favorable for improving the nitridation quality by having an incompact structure after calcination, as a published study deduced that the size and diameter of iron-based precursors have an important effect on the nitridation quality.19 Consequently, the tiny nanoparticles are favorable for Fe3N crystallization. Besides, the effect of calcination temperature on the morphology has also been well investigated in Fig. S5.† It can be seen that the submicrorod morphology can be well maintained in a moderate temperature range, while a rather high calcination temperature will result in the structural decomposition of the submicrorods. For the submicrorod morphology of Fe3N

Fig. 2 The SEM images of the MOF precursors (A–C) and Fe3N submicrorods (D–F) with different magnification scales.

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materials derived from MOF precursors, the calcination temperature has a small impact on the morphology. Based on the EDX results of the MOF precursor (Fig. 3), a probable component of the MOF precursor may be FeC2O4. Moreover, EDX spectroscopy shows the element content of the Fe3N submicrorods, indicating that carbon from ethanediamine exists in the Fe3N submicrorods after the nitridation process. Interestingly, differing from a previous study in which the organic component releases the ammonia gas at a high temperature,14 in our Fe3N submicrorods the presence of carbon is also detected as shown in Fig. 4. The increase in carbon content from the MOF precursors to the Fe3N submicrorods is shown in the EDX spectra. The increase in carbon can be attributed to the ethanediamine, and we can deduce that the ethanediamine itself actually triggers the nitridation, rather than the ammonia gas decomposed from the ethanediamine, considering that the MOF precursor and ethanediamine are in separated porcelain boats. The carbon component in the ε-Fe3N submicrorods effectively decreases the density of the material, as well as improves the biocompatibility. Due to their particular morphology, the ε-Fe3N submicrorods show some interesting magnetic properties, apart from the desirable specific magnetization of 79.73 emu g−1, as shown in Fig. 5. The coercivity and retentivity can reach 379.13 Oe and 19.08 emu g−1, leading to a squareness (MR/MS) of 23.93%. Compared to the magnetic results of Fe3N particles prepared from the sol–gel method18 (Fig. S6†), the coercivity, retentivity and squareness show a quite obvious increase, which indicates that the morphology control leads to a great impact on the magnetic properties, although the specific saturation magnetization is in a similar range. These particular magnetic parameters can be attributed to the magnetic moment distribution

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Fig. 4 The selected area of an SEM image (A), the EDX spectrum (B) and the element content of the Fe3N submicrorods (C), where the elements silicon and gold can be attributed to the silicon substrate and etched gold nanoparticles on the surface of the material.

Fig. 5 The hysteresis loop and initial magnetization curve of the Fe3N submicrorods.

Fig. 3 The selected area of an SEM image (A), the EDX spectrum (B) and the element content of the MOF precursor (C), where the elements silicon and gold can be attributed to the silicon substrate and etched gold nanoparticles on the surface of the material.

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due to the submicrorod morphology and the magnetic exchange coupling behavior from the insulation of trace carbonin ε-Fe3N submicrorods from ethanediamine.13 Besides, the inset exhibits a good dispersity in normal saline. It can be seen that the ε-Fe3N submicrorods can be homogeneously dispersed into normal saline, which implies good MRI properties as contrast agents. They also can be easily absorbed by a permanent magnet from the suspension state due to their excellent magnetism. Although Fe3N has been fabricated for several decades, its application still needs further investigation. The MRI properties of Fe3N submicrorods are first investigated as a potential candidate for the contrast agent. As shown in Fig. 6, the MRI properties of a series of ε-Fe3N submicrorod suspen-

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Acknowledgements

Published on 13 November 2015. Downloaded by UNIVERSITY OF NEBRASKA on 17/12/2015 05:55:18.

This work is supported by National Natural Science Foundation of China.

Notes and references

Fig. 6

Relaxivity time and MRI properties of the ε-Fe3N submicrorods.

sions are measured on a nuclear magnetic resonance contrast imaging analyzer. The samples display a concentration dependent signal drop, implying that the Fe3N submicrorod is a typical T2 contrast agent, which is also consistent with some other iron-based MRI probes. A linear increase of the T2 relaxation rate also confirms that the as-synthesized ε-Fe3N submicrorods are a T2 contrast agent material with the increasing concentration of ε-Fe3N sub-microrods. The calculated r2 relaxivity is about 549.18 mM−1 s−1, which is reasonably higher than that of a Fe3O4 contrast agent (178.30 mM−1 s−1),5–7 demonstrating that the as-synthesized ε-Fe3N submicrorod is an effective contrast agent material.

Conclusion This study offers a modified ethanediamine nitridation to prepare ε-Fe3N submicrorods from a designed MOF precursor. As-synthesized Fe3N submicrorods, with excellent saturation magnetization and homogeneous dispersity in normal saline, are verified as an effective T2 contrast agent material with a good concentration dependent signal and high r2 relaxivity. We believe this study can develop Fe3N as a potential magnetic material.

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Dalton Trans., 2016, 45, 296–299 | 299

Synthesis and magnetism of ε-Fe3N submicrorods for magnetic resonance imaging.

Fe3N submicrorods with desirable magnetic properties were prepared from a designed metal-organic framework precursor via a modified ethanediamine nitr...
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